1. Field of the Invention
This invention relates to an automatic focusing apparatus for a zoom lens.
2. Description of the Prior Art
In conventional zoom lenses the front component generally serves as the focusing component. For example, in the four-component zoom lens illustrated in FIG. 2, variator lens 2, i.e. the second counting from the front, moves along a linear track 6 to vary the focal length of the entire system and effect zooming. Compensator lens 3 moves along in a curved track 7 to compensate for the image shift resulting from variation of the focal length. An independent movement is imparted to the front or first component 1 for focusing purposes. The fourth or relay component 4 forms the image of an object at a constant plane 5. When focusing such a zoom lens, one need only take into account change of object distance in adjusting the axial position of front lens component 1, regardless of the focal length of the entire system. This has the advantage of making the structure of the operating mechanism for the focusing component very simple.
However, the front component 1 is, as a rule, far bulkier and heavier than the other components 2 to 4. Accuracy of control is thus difficult to achieve when automatic focusing is done with focus component 1. For example, because the moment of inertia is large, the usual drive systems operate inefficiently. Also, because the load is large, the speed is very slow.
In another known focusing method for zoom lenses, one of a number of lens components is moved to effect focusing. Position adjustment of that component must be controlled in accordance with not only the object distance but also the focal length of the entire system.
In such a zoom lens, focusing to the in-focus condition is followed by zooming. A necessary re-adjustment in position of the focusing component may be automatically made for achieving the resulting change of position for a correct sharp image. Such a technique is disclosed in Japanese Laid-Open patent application No. Sho 56-162728, a practical example of which is illustrated in FIG. 1. Here the zoom lens is constructed of a front or first component 101 which remains stationary during focusing and zooming, a second component 102 axially movable for variation of the focal length of the entire system, a third component 103 axially movable for image shift compensation and a fourth component 104 for focusing. Focusing component 101 is not moved by the zoom actuator. The zoom components 102 and 103 are moved in differential relation by respective control cams 106 and 107. The focal plane is indicated at 105. The actual axial position of the variator 102 is converted to a voltage signal by a focal length detector 109 in the form of a potentiometer with slider 108 riding on the holder frame for the variator 102. Because the axial position of the variator 102 varies as a linear function of the focal length, one value of voltage at the slider 108 unequivocally corresponds to one value of the focal length of the entire system. A drive circuit 111 for an electric motor 112 produces output pulses whose frequency varies depending upon the magnitude of voltage at the output of the focal length detector 109, so that the speed of movement of the focusing component 104 varies as the focal length varies. An in-focus detector 110 controls the direction of rotation of motor 112 depending upon the far and near focusing states and stops energization of motor 112 when the in-focus condition is detected. The speed of movement of the focusing lens is varied not as a continuous function of the focal length but discretely. For example, two pulse generators of different frequency can be used in combination with a changeover element. The changeover element responds to the output voltage of the focal length detector 109 crossing a certain critical level and renders one or the other of the pulse generators operative. The number of pulse generators may be increased.
When the image shift resulting from the change in the focal length is compensated for by moving either the lens system as a whole, the compensator alone, or a master lens, this image shift compensation may be carried out in an automatic focusing system such as is known in U.S. Pat. No. 3,972,056.